I will need to get a SIM card to get mobile data while traveling in Eibar and in the Netherlands. What carrier should I go to in Eibar? In the Netherlands? One carrier would be convenient, but I can switch if needed.
Any of the mayor carriers in Spain will work. They have shops in Eibar, in one of the main streets. You can go either Movistar, Orange or Vodafone:
I purchased an eSIM from https://www.simoptions.com/. I installed it on my phone before departing, and it automatically activated when I switched my phone to use it upon arrival. It shouldn't matter much which provider, since the EU bans roaming charges.
Be careful that after Brexit some UK carriers may not apply free roaming service all over EU.
Thank you! It looks like Vodafone has a cheap 10 euro prepaid plan.
For others, here is Movistar: